I built a lab for the CCNP BSCI test about two years. I let my certs expire and may renew them. If I have a bunch of 2500 routers, would they still be relavant for studying the BSCI? Also, they are running in a stereo cabinet in the basement with about 1/8" space on each side. Should they be OK or will that ultimately cause problems? The basement stays pretty cool but you could say the circulation is less than perfect.
You can safely stack many 2500s (or 2600s for that matter), but make sure the routers have their little rubber pads attached, which slightly raises them and lets air pass between them. Especially important since the fan is below, and not on the sides. If you don't have the original pads, use something else.
In general I would say that the 2500s would still be relevant for studying BSCI. They can not do some things that would be nice, such as trunking. But they do the functions of routing OSPF, EIGRP and other things that are the main points of BSCI.
I would like to see the routers with better clearance and more circulation. I have found Cisco routers to be pretty forgiving about temperatures. But I would not want to see how far you can push them.