I think so long as you sign AND use verbal to reitterate the WORDS they should be fine. my oldest started signing at 9 mos, we learned a new word every 2 days for almost 3 mos. BY his 1st bday we were signing sentences. I always used verbal with him. we signed though b/c he had no signs of wanting to speak and i was sick of not being able to communicate with him. he didnt speak till his 2nd bday (he said ma-, da-, ee-, THATS IT till age 2). My 2nd son started signing at 8 mos, now at 3 he blows peoples minds with his descriptiveness and proper use of pronouns, verb tenses, etc.

I think so long as a child understansd and CAN communicate it shouldnt matter HOW. We were worried that while my oldest could sign he coudlnt speak. we thought something was wrong with him. turns out he just had nothing he wanted to verbally say

__________________