Firebrand,
There are some downsides to using Y-cables to mix incoming signals. Using Y-cables for outgoing signals, ie one amp out to 2 speakers or 2 headphones, usually works fine, but for incoming signals, the electronics can be tricky and the sound often suffers. First, the Y-cable will connect the mics in parallel, halving the mic impedance as seen by the mixer preamp. This may or may not produce acceptable sound with your mixer. It will change the relative volume of the dual mic channel versus the single mic channel, making it harder to balance the sound of the 3 mic setup. One of the reasons mixers don't use simple parallel adding of signals is to avoid this problem, that is why we use mixers to mix incoming signals. If you want to try it and see if the sound is acceptable for each mic, just make sure you test it at the same gain settings you will be using on stage. You can get a decent quality Y-cable here:
http://www.audiopile.net/products/Mic_I ... heet.shtmlHave someone else play your congas, so you can listen from a distance. Trying to listen to the sound quality of the amplified sound while you are playing is hard because you mainly hear the direct sound from the drums over the amplified sound from the speakers. By the way, Audiopile sells a lot of decent pro-quality sound reinforcement stuff at great prices, EWI is a decent brand, much better than Hosa and house brands sold in music stores.
Even if the sound is ok at that stage, there is another practical issue to consider. If you position your drums and mics so the sound from each drum is at the volume you want, you will still not have the ability to use the mixer to separately adjust the volume or to EQ the different drums. Once you determine the exact mic location that gives your different drums the relative volumes you want, if during the set one of the mics gets moved relative to the drums, one drum will sound louder than another, and you will have to move the mic back to its original location while you are playing. A couple inches can make a big difference. With separate channels for each mic, you or the engineer would be able to simply cut the volume on one mic or raise it on the other. If the tumba sounds better with a little bass boost and the conga sounds better without it, you won't be able to adjust these separately on the mixer unless they are mic'ed separately and go to different channels on the mixer.
If you are using the 6 channel mixer as a submixer to feed one or two channels of congas to the main mixer, you should have plenty of channels to give each mic a separate channel. If your entire band is using the 6 channel mixer, you should consider getting a 4 or 6 channel mixer yourself to use as a submixer. Even if you sing, you can still fit 3 conga channels and a vocal channel on a 4 XLR input mixer. Then you could mix those 3 or 4 channels and send one (congas) or two (congas and vocals) channels to the main mixer. That would give you some control over your own sound and still give the engineer some control over overall levels for feedback control, total balance, etc.