Hi BJR, you can change the email address you use for outgoing emails at any time, I find that, at worst, it may confuse the people receiving your emails from a different address, and if they just "Reply" they will be replying to your gmail account so you need to be downloading emails from that account too, but that's all.
On my iPhone I can chose the sendind ID on a message by message basis quite easily, but if you have a lot of mail it might be more practical to change the default outgoing mail server, I expect that android has a similar default setting capability - you appear to be across those settings.
If you set up and use your gmail account as as your outgoing server, you will have no problems. (there is no problem adding additional email accounts as you are asking, I have 3, but some acquaintances have 20 or so - it just takes a couple of seconds longer to get all your mail)
Your gmail account may be set up, but check that your gmail.com mail is also being checked by your phone.