If your tiffs (including the jpegís you converted to tiffs) have an embedded profile (sRGB or ProPhoto for example), ACR will recognize it. If they donít have an embedded profile, ACR will assume that they are sRGB. In either case, with your settings in ACR to output 16 bit ProPhoto, then all your tiffs (even if they start as sRGB 8 bit) will be output in 16 bit ProPhoto and have that profile embedded. What you donít want to do is open in ACR any tiffs or jpegs in ProPhoto without an embedded profile because ACR will incorrectly assume that they are sRGB and likely give you weird, over saturated colors.
As long as your original tiffs and jpegs have embedded profiles, however, I donít see any benefit in converting them in Bridge. ACR will not change/degrade your original jpegs or tiffs. ACR is totally nondestructive. When you output a modified jpeg or tiff from ACR, you output a copy; the original is still intact.
So, in summary, I donít see any real problem in using the same 16 bit ProPhoto color space for all your ACR outputs. You wonít gain anything by doings so with your 8 bit sRGB photos, but I donít see any harm. Just be aware that for some purposes, like for the Web, sRGB is a better option, but if need be you can always output another version (copy) from ACR in 8 bit sRGB.