Is there a difference in how the above groups were looked at and treated? Many films depict SS soldiers actively hunting for Jewish people, checking census registers and such to find them and eventually murder them.
Were similar behaviours carried out against the Romani (who often did not live in traditional housing and may have been 'easier' to identify?) and the slavic people of Eastern and central Europe (some of these were of course also Jewish). Or did the crimes against the slavic peoples come about as a necessity of territorial expansion which then 'necessitated' ethnic cleansing and genocide?
It's important to remember that anti-Semitism was central to Nazi Ideology. Nazi Ideology was the culmination of Centuries of German (and European) anti-Semitism that saw Jews as "Hostile Aliens" destroying the German nation. Thus you have point 4 of the Nazi party Platform:
This entire document only refers to Jews. No reference whatsoever is made to "Romani/Gay/Disabled/Slavic. Etc. Similarly. Hitler's political testament would have him blame the entire war on "International Jewry". Again, no mention of other groups. Nazi Ideology was founded on Jew hatred.
Ideology naturally reflected itself in Policy. At first, Communists and Political prisoners were persecuted and tortured as enemies of the state, but they weren't systematically exterminated. It's also important to note that only German (and Austrian) PWD were targeted In action T4, because going back to the Nazi platform, the objective was to keep German Blood "pure", so T4's purpose was to remove "impurities" of the German Race. When WW2 started and large numbers of European Jews fell under German domain, the Nazis' ideology began to take precedence. Thus, we have documents like the letter from Willem Hagen, the Chief Nazi medical officer in Warsaw. In this letter, Hagen asks Hitler for Authorization to "Treat the Poles (Younger than 10 or older than 70) the same as the Jews, that is, to kill them".
In other words, the Jews were the main targets of the Nazi killing apparatus, in accordance with Nazi Ideology that saw them as the main "threat". The Romani were similarly targeted, in that many stereotypes about them ("Vagabonds", "Criminals") also applied to the Jews, but more importantly, they were also seen as "Aliens" and a "threat" to the German Race, like Jews. So, in Eastern Europe, Nazi death squads killed Romani with the same ferocity as they did Jews. It's important to remember that Nazi massacres often happened because the Nazis were racists and thought that the extremely poor and religious Orthodox Jews of Eastern Europe embodied their worst stereotypes/fantasies about Jews. They felt the same way about Romani. There are Wehrmacht Records for example that record how, in "reprisal" actions, the Germans often killed Romani as often as they killed Jews, for no reason other than their "Race".
First and foremost, Nazis were racists. They killed Jews because they were Xenophobes and thought Jews were "Aliens" because of their alleged "strange customs" and "Alien behaviour". This also applied to the Romani. Thus, both groups were the main targets of Nazi policy. The Nazis were also Racist against Slavs and killed Many of them, but they were a bit higher on the "Hierarchy", as the Nazis imagined that Slavs could have a place as servants/slaves of the "Master Race", which was why we have documents like Hagen's letter, which explicitly state that the Killing program was for the Jews first, and that permission would be needed to kill allegedly "unproductive" Poles with the Nazi killing apparatus. Disabled/LGBT were targeted because they were seen as "impurities" of the German Race, so more often than not it were German/Disabled LGBT that were killed, and only rarely others. Nazi Ideology was racist, and their racist world view saw Jews and Romani as "Aliens" that needed to be killed, first and foremost.
Willem Hagen's letter. http://holocaustcontroversies.blogspot.com/2019/08/the-hagen-letter-to-deal-with-13-of.html?m=1
Nazi party Platform https://www.historyplace.com/worldwar2/riseofhitler/25points.htm