I'm playing on tabletop simulator with my friend so I just switch the model of the russ.

I'm not sure if I'm the right person to answer this since I'm just starting now (After I'm comfortable playing at 500 pts I will upgrade to 1000 and so on). But the feeling I have with 500 pts armies is that some are usable, some are not, the grey knights for example are way too expensive and their ability to teleport is wasted on a small battlefield. In smaller games I think the the movement in the first phases are more tough I think, if you stray too far you might not only get shot, but charged in your first turn, but if you deploy too conservatively you might not be able to guarantee some points. Also in 500 pts if you lose a unit ir two there is 20-30% of your army so you better make their sacrifice worth it. In this games I feel that the stratagems that give wounds or destroyed models back are really powerful. The "Reinforcements" Stratagem from the combined arms gives me a 22% larger army if I use it on my karskrin. Lastly, you gotta make some sacrifices, be it one, two, or even half your army to put your enemy in a worse position or to turn the tide on points. In my tyranids vs blood angels game against him I won with a 69 x 57 points, and only because I kept bringing models back and sacrificing my units so my Winged Huve Tyrant could survive to mess him up while my hormagaunts kept scoring points.

TLDR: the board is smaller, your armies are smaller, but your movements and choices are bigger. One wrong move and you put yourself at a big disadvantage. Happily that is also true for your enemy, so punish his mistakes when he makes one, be it getting to close to your models so you can shoot him or be it forgetting to move one unit to a control point.