I haven't found that to be the case. The biggest challenge facing ag pilots who begin in SEATs is mountain work, but it's something they learn. The government places extremely heavy emphasis on radio work, such that fire schools are mostly about communication.
I find that leadplanes seem to have a hard time operating at the correct altitude, nearly always passing high on the target, forcing aircraft doing the drop to workin the lead's wake. Then the lead complains that the drop was too high, if the tanker stays above the lead's wake. I can't think of any lead/ASM pilots presently that have tanker experience. Or ag experience.
Your lack of ag or fire experience isn't necessarily an impediment. It depends on your background and experience. You'll usually need to have a turbine tailwheel background. Mountain experience is necessary.There aren't many 802's on floats and operators that were considering them have moved away from that direction, given problems with the fireboss. Airspray is about your only choice in the lower 48 for that. You might try Aeroflite is you have multi-sea experience, for the CL415.