The rules for picking allied contingents are found at the start of each list book. They were refined a bit along the way, so if possible refer to Bk 4 (the latest-published list book) for the clearest and most definite version.
The short version is that an allied contingent must include between a quarter of the minimum and a third of the maximum of each compulsory troop type, and cannot include any non-compulsory troops unless either list contains a statement otherwise.
A Palaeologan ally would thus consist of:
1 general
1 kavallarioi
2-4 stratiotai
2-8 archers (you can of course not actually take all eight, as that'd take you above 11 elements)
1-2 tourkopouloi
It can also have 0-2 baggage, because they're specified as per general, and such troops are also allowed by the allied contingent rules. Note that baggage don't count against the 11.
(That's a Palaeologan ally for Catalan Company. At later dates, the tourkopouloi may not apply, and at earlier ones you may have to have Cumans.)
