Fine, steal it if you can but be aware it's an incredibly closely guarded secret, ludicrously so. But if you pull it off then fine but believe me I'm going to make you work for it and even then you will likely still fail. Be aware though that if my guys manage to trace the theft back then, well, it could get very bloody. Oh though you have given me an idea for an excellent counter intelligence op. 

As for the extent of the ISI operation it seems pretty huge already, though admittedly it involves non standard military intelligence things, such as a large RnD wing. For the sake of balance and keeping a decently flowing RP going I'd recommend that you assume all developed nations have a pretty decent counter intelligence machine, though not necessarily a great intelligence gathering machine, if you see what I'm getting at. So the ISI will succeed occasionally but not reliably, especially not with really important things to the other player. That would have to be worked out on an add hoc basis. Otherwise it starts getting like mind reading in smaller scale role plays. I've seen people just walk away from these sorts of things before because spies knew their almost every move so nothing they tried to do had any real point. 

So, ISI probably have a few operatives in every other nation but they might not be able to do a huge amount more than get low security stuff or things you just can't keep secret (Like large scale troop movements in inhabited areas) and then you have to think about how you get the message back home. In reality the running the message back bit is what tends to fial the most and get spies caught, as far as I'm aware any way. As for more advanced things like moles in secure wings of the military it might not be a good idea to assume that at the start. More ambitious operations to get high security stuff, kill high ranking figures, sew civil unrest would enjoy a much lower success rate or require real effort and don't be surprised if ISI agents in foreign countries get their doors kicked in and shot on a seemingly random basis. 

As for their actions in war, behind the lines sabotage etc I'd recommend a similar approach of "they can do it but randomly and unreliably." 

Hopefully that answers your question.