The Delta66 has 4/4 analog I/O and 2 channels of SPDIF. If you want to record 6 channels at once, then you're going to need to be recording at two SPDIF devices, or a stereo SPDIF pair from a mixer with SPDIF outputs.
If you're recording instruments direct, then there are plenty of DI type boxes like the POD Pro and J-Station that have SPDIF output.
You can't plug any instrument, be it a mic or a guitar, directly into the analog inputs of the Delta because they are line inputs. You will need preamplification of some sort. Most mixers have mic pres and there are some pretty cheap but usable standalone preamps like the ART Tube MP and the m-Audio Audio Buddy. Preamps do have an impact on sound quality but you're just getting going so it's the least of your concerns.
A digitial mixer would only be a good solution if it had per-channel analog outputs...like channel inserts would work. If it doesn't have that, then it would only be useful for your single SPDIF pair, and would basically be overkill.
The best solution, if you really need to record six tracks at once, would be to look into something like the Delta 1010LT which is a "light" version of the 1010 (no breakout box, etc). It has 8 analog ins & outs, and two of those inputs have mic pres. It also has 2 channels of SPDIF and I believe it retails in the sub $350 range. If you got a 1010 or 1010lt you could use any combination of mixer, di boxes, preamps, etc to get the full 6 channels you require (you'd actually have two more analog to play with and two additional SPDIF if you ever get SPDIF equipment).
Slackmaster 2000