Yeah, a school of spilos will be great provided they dont kill each other. Personally I would suggest getting 4 for the 90. Just get small ones to start with. Preferable 2" or smaller. Then as they grow you will have a better chance of at least 3 producing a good school. With 4, if one dies, you will still have 3. If you start with 3 and one dies you will have 2 and quickly one. Perhaps 4 is not idea for life in a 90, however, if all 4 establish a school that works in the long run you can sell one off.
Thats what I would to ensure you end up with 3 in the end.