ありがとうございます　－　ありがとうざいます
Hello there! When saying 'good morning' or 'thank you' (formally), most people don't pronounce the 'go', or barely.

e.g.
ありがとうざいます (arigatouzaimasu)
おはようざいます (ohayouzaimasu)

I understand this is only used in speech, but
1) is it less formal?
2) does it sound more native?
3) when to not use this? (or is it always okay?)

Thank you!
よろしくおねがいします。

Question

ありがとうございます　－　ありがとうざいます
Hello there! When saying 'good morning' or 'thank you' (formally), most people don't pronounce the 'go', or barely.

e.g. 
ありがとうざいます (arigatouzaimasu)
おはようざいます (ohayouzaimasu)

I understand this is only used in speech, but
1) is it less formal?
2) does it sound more native?
3) when to not use this? (or is it always okay?)

Thank you!
よろしくおねがいします。

Answer

I've never listened(noticed) those sounds though mika-san wrote "Some Japanese people surely speak like those". I think that maybe you can not listen to/catch "ご(go)" correctly. You wrote the next.
ありがとうざいます (arigatouzaimasu)
おはようざいます (ohayouzaimasu)
But It's strange that the former part is pronounced strictly and the latter part is pronounced negligently/lazily. ありあとあす［əriətoəs］, おはよっす［ohajoss］are could be listened. I listened to them.

Answer

1) Some Japanese people surely speak like those though, I can say with confidence that is no good way of speaking in formal. 
2) I don't think that sound more native than ございます。 Those are just different way of speaking.
3) I think that most of Japanese people don't use that even if it is in casual. 「ありがとう」 and 「おはよう」 are more common in casual. I hope my explanation makes sense for you!