Ok so we have a nice system to design your own very powerful summon, the succession shop (lady in blue dress standing on crystals).
1. You can take one skill from summon and put it on empty slot of another summon for 20 gold (summon providing skill is destroyed).
2. Or you can remove one skill from a summon for 10g. The game states it should be used mostly for [!] skills, which are not beneficial, but to make a dedicated summon with a good skill selection you will need to remove good skills too if they are not well suited for a given girl form.
3. You can also unlock any skill instantly with a skill coin. This is for skills that otherwise require using the summon to farm hearts.
Finally in one of such shops later in the game you can buy a copy book for 100g. Equip a summon with skills you want to copy and then equip copy book and all unlocked skills of that summon will be on the book, which you can then use for first action without worrying about losing unique summon with a nice skill you want on your designed summon.
With proper skills final battles are quite easy, the pink bunny without an arm was the only one that provided some challenge, last one was much easier for me.
For girls having fun with themselves it seems gloves you get from progressing main quests are not enough for all girls. Some of them do have down+x animation, but sometimes the skill shows up in status but it's not working and sometimes even doesn't show in status. This is independent of any skill changing summons that may be or not equipped. Are there any additional requirements to make it work?