As a regular visitor to Italy, here is my advice.
Whether to go in January
I think it's a great time to go. Both cities can be absolutely full of tourists in the warmer months of the year, so winter is nice time to go. It is likely to be overcast sometimes and may be raining or foggy, but when the sun comes out it's lovely. Rome and Venice can be photogenic in any season, and I personally find the stark light and shadow of the midday sun the most difficult to deal with.
Rome and Venice?
Once you accept that you're not going to see everything in the cities, I think it's possible to do both in a week. It would be much better to travel between the cities by train than fly. The high speed train La Frecce takes about 3 hours 30 minutes to do the trip and it's city centre to city centre. I just checked on the official train booking website
Acquista il biglietto con le nostre offerte - Trenitalia and it looks like it costs about 50 euros. Another nice thing about the train is that you get to see the countryside while you travel, which is very nice between Florence and Rome.
Safety
There are pickpockets around the tourist sites and on some public transport, so you have to be conscious of that. Just avoid wearing luxury brand goods, carry a nondescript bag and put your camera away when you're not going to use it for a while, and especially on public transport.
Lenses
My ideal kit would be the DA 16-85 and a "DA 24 f2" for the night shots. But since that last one frustratingly doesn't exist, I would take the DA21 or the FA35 instead.