Black and White are the only two colors in 651 that can come in matte. Q1: Matte is typically easier to read because there's no glare.
Q2: If they want glossy gold, but matte black, give them what they want. The customer is (most of the time) always right - whether they have bad taste or bad sense or not. Different strokes for different folks (and whatnot).
Q2a: I'm not sure what you were planning on charging them in the first place, but if you charge them enough that covers the cost of your materials then you're ahead of the game - buy some rolls of the vinyl that you need, for the size that they're looking for, and you get to keep the rest on the roll. I'm a hobbyist and do a commercial project now and then. Sometimes I just charge for the materials of a new color, and smidge more for the app tape and my time. If have the color in-stock, then I just charge them whatever I would normally do - with exceptions for family, friends, and difficult customers. Know your hard cost and go from there. There isn't a commercial job that I will do that will ever be under 0.05 per square inch <- That'd be my wholesale price.
Q3: Matte might need a higher tac app tape than the gloss. Depending on the size of your existing app tape, you do not necessarily need to buy bigger tape, just overlap the edges of the tape until it's all taped up ... unless you're cutting huge and only have a 6" roll, then save yourself time and buy bigger.
Q4: You don't state what machine you have, so if tracks well, then you can do the black in one shot. If it doesn't track well, then you might want to cut the words separately and use blue tape on the wall/glass for alignment.
This is just one girl's opinion on it, and I'm sure others have differing opinions on what to use, how to use it, and how much to charge.