Not sure about your fit questions - but as for the others
I wouldn't bother with a bottle. A 20 min race isn't long enough for you to really need to drink. Most people don't even have cages on their cross bikes (it interferes with picking up and shouldering the bike), much less carry bottles. Our ace cross racer only missed having liquid once - it was a long race 45min or an hour? (she races with the pros) and it was hot, like in the 80's, which is highly unusual for a cross race.
The bike hurting your shoulder - yes, you can get a little triangular pad that goes between the seat post and top tube. Our good cross racers say don't do it... your shoulder will break in and stop hurting the same way your back side does with a new saddle. If you get a pad it will never stop hurting. If you only plan to do one or two races a year - then it might be worth it.