I would be very careful about putting stickers on perspex. The solvents in the glue might (or might not) impact the perspex, but you don't want to find that out after the fact.
Also, on composite aircraft, if you use dark stickers this may cause local heating, which reduces the strength of the composite. There's a reason that composite aircraft are white...
Oh, and don't put stickers on aerodynamically critical bits of the aircraft, such as the wing leading edge, or anywhere where it can impact a mechanical movement of something, but I guess you guessed that already.
Personally, I would insist that my sponsor (because I guess that's the situation here) would use aviation grade vinyl to print their stuff on. Essentially the same as what is used for the aircraft lettering.