Impulse Tracker will have issues with files exceeding its originally specified ranges, however most other players have no problems with the following:
- 256 rows per pattern or more (OpenMPT and I think XMPlay cap this at 1024)
- 240 distinct patterns (could in theory be 254, but is less for historical reasons, I guess)
- 200 samples (could again be 255, but this is probably another historical limit)
For your SPC conversion, you could probably save a lot of patterns by varying the ticks per row (Axx effect) depending on the number of events. Alternatively, use the SEx or S6x effects for repeating a row or adding ticks respectively.
Also, it might be possible to use 2 ticks per row instead of 1 tick at the cost of some possible minor precision loss, by using volume and tempo slides (D0x decreases volume on all ticks but the first, Dx0 increases it, Fxx with xx < E0 increases pitch on all ticks but the first, Exx decreases).