Dear Dave001776,
The most complete line of True 15mm figures for the SP-AM War is "Rough Riders!" from TVAG's Splendid Little Warriors, complete with the Black troops of the 24th & 25th Infantry, 9th and 10th Cavalry, and dismounted Cav figures for the US 1st volunteers, etc. These are bargain priced and provide all that you can use for Cuba, including Spanish troops of all kinds, with Naval Infantry as well.
Here is the direct link to see for yourself: link
Regarding rules for the battles, most rules writers have not really a clue how to represent the seeming "one-sided" actions that characterize the two major fights in Cuba. Basing rules on standard gaming models (frequently either ACW or WW I sets), they completely miss how nearly both San Juan Heights and El Caney came to being US disasters due to total inexperience and poor training (with fully 10% US casualties suffered from forces engaged--almost as many as the total number of Spanish Defenders).
May I encourage you to look at the rules, "Rough Riders!" that present the genuine factors that determined these battles, which were not numbers or fire power. When the Yanks win, it's because they EARN the victory, not just pick it up off the field.
RR! is card driven with a specially printed full color deck that provides the exact ID of each unit--Spanish and US--and which determines the order in which their movement and fire are adjudicated. With only a few historical exceptions, every Unit's actual CO is named and "game rated" for his abilities.
The moral and physical effects of Parker's Gatlings are a key part of the game--if he can get them into action! Detailed, scale maps of the San Juan Heights and El Caney Battlefields allow players to play on the actual terrain with line of sight and other issues never considered in other rules.
The rules even come with special full color flags to help ID the US units for all actions in Cuba (as locating the CO's of the specific units is crucial to the game system).
You may read more about the rules through this link
TVAG also provides authentic 15mm card models of the key buildings and Block Houses that so heavily influenced these battles.
Sorry for the "hard sell" sound, but the battles in Cuba are not well understood and a lot of unnecessary myths have sprung up--and most of them are detrimental to the actual accomplishments of the participants on both sides.
I can't offer any help regarding compatibility of our True 15's and Freikorp's, but I suspect they'd be close, at least.
Best of luck however you go,
TVAG