From experience, Tamiya Acrylics are great for airbrushing even straight from the bottle, though I would suggest that you thin it a little specially when doing detail work. For handbrushing, they're not the best if you spend more than 5 minutes in a brush job. Reason is the paint dries too fast causing a surface latex-like film to develop in the bottle, which tends to stick to your brush as you dip into it. This can get into your model at the worst possible time
. Like Scott recommends, I add a few drops of acrylic retarder to my Tamiya Acrylics to prolong the drying time. I used to exclusively handbrush with enamels but since I discovered retarders, I'm also using acrylics now just as much.
Yes, it is okay to handbrush acrylics over enamels, and vice versa, PROVIDED that the underlying paint or paints are fully cured (24 to 48 hours, I think more if gloss paints are used). I've had success so far with this combination. Acrylic-over-acrylic is also okay. Funny thing is, handbrushing enamel over enamel can sometimes cause the dried paint to dissolve and mix with the new paint (GRRR!!!), even if sufficient curing time was given. This may be cause by various factors such as the thinners used or the paints themselves, as well as temperature and humidity which can prolong the drying time. To avoid this, nowadays I spray or brush on a barrier coat of Future to protect the underlying paints from being attacked by new coats of paint as they are being applied.
Good luck to your efforts!