Well, it sounds like you're playing on the same rig I am (except I'm using the Wick Classic 3XL). Mostly I'd say that your only problem is practice and experience. 6 months isn't all that much time on the instrument. I find the embouchures on euph/tuba/trombone to be VERY different. Tuba permits you a lot more latitude (and more latitude for a sloppy embouchure in mid and low register). I suspect that you're still a bit too tight with the embouchure in the mid and low range. And maybe your mouth placement on the mouthpiece is not quite right. Experiment with it, and, of course ....... long tones. Work on the mid stuff first, then the low range, then worry about the high -- which you already should have covered since you're coming from euph. Do NOT use too much pressure on the mouthpiece. The embouchure needs to be more relaxed than you're used to.
Pedals: So what do you mean by this? The first genuine pedal on that horn is the open Eb that is the SECOND Eb below the bass staff (5th space below the staff). You almost NEVER need those notes. Honestly, the true pedal C is about the low limit of my range. So I'm assuming you really mean the octave above that (i.e., in the first octave below the bass staff)? Anyhow, using TOP lip for low range is not something that I think a lot of (any) tuba players do. Absolutely can't make it work for me. Try moving the mouthpiece down and using mostly bottom lip for the low range.
In terms of the mouthpiece, I strongly recommend you don't mess around with alternatives until you've got more time in, and better results. I spent a HUGE amount of time experimenting with all kinds of mouthpiece for this horn, and the Wick 3L mouthpieces are in my experience the best. They were really designed for this instrument. At some point you might want to try the 3XL. The rim is more comfortable and it gives the impression of being a bit wider (but just because of the rim contour). It takes a little more control in places, but I like it a lot better than the 3L, especially in the low range. However, what turns out to be the best mouthpiece FOR YOU depends a lot ON YOU -- and in particular on the size/shape of your mouth, lips, etc. But you should be able to get all the range you need for your local orchestra. That being said, it is now pretty unusual to see an Eb tuba in an ORCHESTRA. The Brits have played them in orchestras for a long time, but I think they're giving way to more CC horns. Even Fletcher tended to use a CC for certain orchestral work.
Gary Merrill
Wessex EEb Bass tuba (DW 3XL or 2XL)
Mack Brass Compensating Euph (DE N106, Euph J, J9 euph)
Amati Oval Euph (DE 104, Euph J, J6 euph)
1924 Buescher 3-valve Eb tuba (with std US receiver), Kelly 25
Schiller American Heritage 7B clone bass trombone (DE LB K/K10/112/14 Lexan, Brass Ark MV50R)
1947 Olds "Standard" trombone (Olds #3)