I have the same advice as everybody. However, if you do not want to invest in a good set of congas, perhaps dealing with the heads could solve your problem temporarily. The problem with the sizes of Headliners is one you have to deal with. Remo is coming out with a NuSkyn, one of their synthetic heads, in sizes in the "entry level". I do not know if they are available out there yet. (There is a mention of it in their site
http://www.remo.com).
Congas that are quintos come in 11 inches. Not that many people buy quintos though. (Normally everbody buys the standard big sizes 11 1/2 and 12 3/4, I think that is correct). You might want to special order. Another alternative is to have them "mounted" by somebody else. (Depending on what location you are it could be tough to have somebody mount them for you or selecting the sheet of "hide". If not, ask some players in your area). For the 12 "tumbador" the best advise to give is to contact a Meinl distributior and see they could get you one of their "good heads". (Meinl has some Marathon series congas in 11, 12 and 13 sizes. Their "factory" heads are pretty good)!
Regarding the slap, it takes time finding "your slap". Good thing you are doing is taping yourself. Watch what others do. Ask their advise. Tell them to show you. Show them what are you doing. The technique for the slap is "standard", however, what works for some might not work for others, i.e, area where you triying to the slap, angle of your hand, etc, etc. Look for "your spot", following the right technique so your hand do not suffer. Practice, practice. You have to develop also what I call your "weight" with your hand that should work with your tuning. Once you find that combination, you will get a "slap" you are content with and eventually you will develop "strength" in your hand and you will do that slap anywhere in any conga and with any tuning.
Do it slow and find your sound!
Good luck!