How do I prepare for a defendant who has several character witnesses lined up to testify on his behalf during the guilt/innocence phase.........?

First, keep in mind evidence of specific acts of good character are not admissible. Schmidt, 449 S.W.2d 39 and that the testimony must relate to a pertinent trait of character. Wiggins, 778 S.W.2d at 893 and Valdez, 2 S.W.3d at 520. Figure out a way to remind the jury "each case should be tried on its merits alone and not on the merits of the [accused]", Roberts, 276 S.W.2d at 578. Otherwise, it seems to me you focus on the evidence of guilt and maybe remind everyone that any of us is probably capable of anything and that you really cannot always judge a book by its cover. Its an interesting question. Of course you also gather anything that could be used to rebut the defendant's portrayal of himself.

Is there any limitation on how you rebut the evidence .............? I was always taught that you didn't go near the Defendant's character at G/I phase, but seems unfair that he can bring in everyone on the block to talk about what a great guy he is and you are limited in rebuttal.

Is his character ("He's one of the best employees I've had in 20 years...") even relevant at G/I ?

You should look at Herasimchuk's book Texas Rules of Evidence Handbook, 253-58 (4th ed. 2001), The character trait has to be relevant to the crime charged. (citing Holt, 912 S.W.2d at 301-02; Mowbray, 788 S.W.2d at 668; Wiggins, 778 S.W.2d at 893; Spector, 746 S.W.2d at 950.
It also has to be a real trait of character. (citing Valdez, 2 S.W.3d at 519-21 (non-pedophile not a character trait) but see Brazelton, 947 S.W.2d at 650 (not a drug seller was a character trait). Being a good employee wouldn't seem to be either relevant or a true character trait.

She also has several pages on responding to this king of evidence. Rules of Evidence Handbook 253-85.

Keep it simple- make the def's attorney adhere to rules 404 and 405... It must be evidence of a PERTINENT character trait to overcome the presumption of inadmissibility... Then, if the defendant gets past that hurdle and gets it in, your rebuttal can be pretty wide-open... I had a case recently where a character witness testified that the defendant, who was his nephew, should get probation rather than prison because, in spite of his actions indicating otherwise, he really was a "good guy," etc. So that allowed me to ask him if his "good guy" opinion would be different if he knew that the 34-yr-old nephew had been having sex with the nephew's 16 or 17-year-old niece while the nephew was married to someone else.

The point of all that is, if you've got something devastating to use against the defendant, the more leeway the judge gives them to talk about how good he is, the more range you should have to ask those "Were you aware of ...?" questions on cross-examination of the character witness.