Hi,
I did combined BF and FF for both my kids from about 6 weeks old, but when it came time to fully wean, I started my oldest on a sippy at 5.5 months and she loved it, never used a bottle again. However with Spencer we had the same issues pp mentioned in finding the right kind of cup, the nuby soft spout was the best one for her.
For the solids, since it is more for practice and fun at this age could you introduce things like pears, prunes etc that would help keep things moving as it were? (Formula really constipated Spencer, we did a daily dose of prunes mixed with banana)
Oh, and he is the same size as Spencer and for awhile she could pack away 8-9 oz in a feed (6-8 months old) but now she tops out at 7 oz per feed. We recently discovered too that she prefers the flavour of a different brand than we usually buy, so we switched and she drinks much more eagerly.
HTH
Heidi