For example how likely would it be for a German squad to get its hands on a few extra mp40s or pistols and for those to be carried by the soldiers, or a British soldier to capture a pistol or sub machine gun and carry it in combat? I've heard of American units getting a few extra BARs or a British tank commander getting a luger and keeping it loaded 'just in case', but I have no idea about how common it was or if the germans did the same.
I am doing a few scale models, some come with soldiers but sometimes weapons snap or get lost, and some dont get used and end up as spares. Basically just want to know how far I can get away with giving non standard weapons to soldiers while still being fairly historically accurate.
It most likely depends on the year, place and country. Soldiers using non-issued equipment was most common in times where supplies were low and ammo was scarce, when the situation was not suited for their standard equipment, such as street fighting with a full-sized rifle, or when captured enemy equipment proved to be superior to the standard issued equipment.
For example, there is recorded footage and photographs of German soldiers using captured PPSh 41 submachine guns in and around Stalingrad and the larger Eastern Front. So many were in fact captured on the Eastern Front that the Wehrmacht started to convert them into using 9x19mm parabellum rounds and MP40 magazines. Others were left unconverted and were supplied with the 7.63x25 Mauser rounds, which were almost identical to the 7.62x25 Tokarev rounds used by the Soviets.
Now these PPShs were redistributed amongst the Wehrmacht and thus became issued equipment, complete with German manual and designation. But many of these guns would have been used as battlefield pickups before being actually recorded and accepted within the inventory. The environment in which the Germans found themselves inside Stalingrad was well suited for submachine guns.
Other photographs of soldiers using non-issued equipment exist, such as Soviet soldiers carrying MP40s. There is also recorded usage of captured Panzerfausts being used by the Soviets, who used them against cover and buildings during street fighting at the end of the war, and by the US 82nd Airborne Division, who found them to be more effective than their bazookas and even dropped with them during Operation Market Garden.
Getting exact numbers can difficult, but it seems that especially the Germans were keen on reusing captured equipment, which was not limited to small arms but also includes tanks and other vehicles. They also manufactured small-arms of captured nations and destributed them amongst rear-echelon troops or reserves. Browning Hi-Power pistols continued to be manufactured by the Germans in occupied Belgium and Krag-Jørgsen rifles and m1914 pistols continued to be manufactured by the Germans in occupied Norway.
Battlefield trophies such as pistols were common amongst all militaries of the war, with Lugers, Walthers and Nambus being sought-after trophies by allied soldiers.
Your main concern when arming your soldiers with non-issued equipment should be ammunition. If your soldier does not have access to a large pool of ammunition he isn't able to use the weapon for more than a few engagements. The Germans were lucky for example that 7.63x25 Mauser was already integrated in the supply chain and ammo pool of the Wehrmacht, which allowed them to more easily acquire ammunition for the captured PPShs.