This is a good question (and strange for me, thinking of something 20 years ago as "history"). The short answer is: the fear wasn't about the Russians, it was about reunited Germans.

Let's dig into it.

I think you that you first need to resolve why the Soviets hung on for 5 years after the wall came down. There were several reasons for this, but most were logistical.

1. The Czechs did not want Russian troops using their railways. So most equipment needed to be transported by sea.
2. The collapse of the Soviet Union in 1991 disrupted plans to house the 560,000 troops returning home.
3. Russian troops took 2.5 million pieces of equipment back with them. In one case they even packed up a concrete runway.

Many Germans and Russians expressed skepticism that the withdrawal would go smoothly. There were fears that the Russians wouldn't leave, or the Germans would pelt Russian soldiers with rocks and other missiles. None happened.

If there was ill-will, it was on the Russian side. In 1994, the New York Times reported the "[Bitter Goodbye] (http://www.nytimes.com/1994/03/04/world/bitter-goodbye-russians-leave-germany.html)" that was the departure of forces from Germany.

A Russian officer was quoted:

We're leaving because we're under orders to leave, but none of us want to go. I won't have a place to live when I get back home. The Germans killed millions of our people and burned a third of our country, and now they're all rich and we're being kicked out like dogs.

The Baltimore Sun reported that the Russians --and this is really interesting-- were "leaving behind hard-currency salaries far more bountiful than what they will earn in rubles in the mother country."

Side note: that suggests to me that the Russians forced East Germany to pay the salaries of Soviet troops stationed there. That'd be consistent with the harsh reparations the USSR imposed on East Germany after the war, but I would need a source to support.

The East Germans were reportedly impatient with the pace of Russian withdrawal as early as 1990:

For the 28,000 people in this town 30 miles northwest of Berlin, 45 years of living with a Soviet military presence that roughly equals the civilian population has been more than enough.

Last month about 10,000 citizens, angry over having their sleep disturbed by dawn-to-midnight flights by Soviet warplanes, marched on a Soviet air base here. About 100 broke through a barbed-wire fence and stormed onto a runway to demand that the base, just one of several in the area, be closed.

[But it was also reported] (http://www.nytimes.com/1994/03/04/world/bitter-goodbye-russians-leave-germany.html):

"We are all very impressed with the professionalism the Russians are showing," said Helmut Domke, who is overseeing plans for conversion of Russian bases to civilian use in Brandenburg, eastern Germany's largest state.

"For three years they have been sending 120 train cars a day on trips that are hundreds of kilometers long," he said. "They have moved 700,000 tons of munitions and explosives without a single mishap. It's their last assignment in Germany, and as a matter of military pride, they want to do it well."

So that gets to your question, about unified Germany's fears over having Russian forces in-country.

Reunification happened on Oct 3, 1990. That means the Soviets were only the Soviets for another year, until Dec 25, 1991 when. And during that time, they were actively removing forces per the Two Plus Four Agreement.

Per the TL;DR answer, if anyone was worried over German reunification -- it was the allied powers. In fact, [Margaret Thatcher was vehemently opposed to German reunification on the grounds that it would weaken Gorbachav] (http://www.spiegel.de/international/germany/germany-s-unlikely-diplomatic-triumph-an-inside-look-at-the-reunification-negotiations-a-719848-druck.html). She was very much in favor of keeping Soviet Forces in East Germany. She told Gorbachev plainly "we do not want a united Germany."

[The worry was was enough for Thatcher to telegraph to the French] (http://www.telegraph.co.uk/news/worldnews/europe/germany/6480413/Margaret-Thatcher-was-horrified-by-the-prospect-of-a-reunited-Germany.html): "France and Great Britain should pull together today in the face of the German threat"

The [French, the Poles, the Israelis, the Italians, and the Dutch] (http://www.telegraph.co.uk/news/worldnews/europe/germany/6480413/Margaret-Thatcher-was-horrified-by-the-prospect-of-a-reunited-Germany.html) also expressed worry about a pan-German revival, the alldeutsche bewegung that pulled Europe into World War II in the first place.

The Americans were more sanguine. We didn't have the European worries about German expansion, and we also still had some 200,000 troops bedded down in Germany (England did as well, not that many).

Condoleeza Rice, who was Senior Director, of Soviet and East European Affairs in the National Security Council in the George H.W. Bush White House, said that

For us, the question wasn't should Germany unify? It was how and under what circumstances? ...

We had no concern about a resurgent Germany, unlike the British or French. If you were going to have a Europe that was whole and free, you couldn't have a Germany that was divided. So, with the possibility that Soviet power was going to be receding from Europe, it made perfectly good sense to try to achieve reunification on terms that nobody would have thought thinkable, even four or five years before.

So the TL;DR -- The Soviets were largely seen as a declining power, begrudgingly accepting their fate, leaving Germany with their tails tucked. But old attitudes about a Germany responsible for two World Wars still prevailed.

There were no NATO worries about a retreating Russia, but there were ample concerns about a resurgent Germany.

First off, the Warsaw Pact disbanded in July 1991, three months before a little less than a year after Germany formally unified, but well before all the soldiers withdrew. Secondly, the Soviet Union itself dissolved in December 1991, so the forces remaining in 1994 were Russian, not Soviet. People weren't alarmed by the presence of Russian soldiers within Germany because the border jumped the soldiers, not the other way around. Moreover, Soviet troops had been in East Germany throughout the Cold War, so their presence was nothing new. The USSR maintain several large, permanent bases in East Germany just like the US did in West Germany. In 1991 there were still close to 350,000 Soviet troops in East Germany, and thus they (and later the Russian Federation as the USSR's successor state) were given until the end of 1994 to remove their forces from Germany in the Two Plus Four Agreement.

[edit] mixed up my dates in my first sentence.