you could simply buy a few new cogs. maybe a 20 and a 22t. Since your fixie is what you will be riding most and I assume is what you ride every day, it would be best to change the gearing up on that if you don't have rollers. start out with the 20 tooth and just ride. You will spin out on flats pretty quickly but you will eventually get comfortable with it. Don't avoid hills and tell yourself not to use your brake. after a couple weeks when you feel more comportable change the cog to the 22t.
My first fixed gear was at 88 GI so I didnt know what spinning out was untill I got my Steamroller. Stock is 66 GI. I could barely ride the thing on flats without spinning out and slaming on my brake, but I used to it after a couple weeks. Whe I started to spin out I automatically moved forward a bit and just barely got out of the sadle. Since I was hopping in the seat like you getting out of the saddle a tad bit helped after doing this for a couple weeks, I was able to sit on the saddle without hopping up and down. This may help you....
I put on a 22t for a hill climb time trial and I spun out on the flats with this until I got used to it. Again dont whimp out and use the break keep on spinning. on flats I can spin the 22 tooth at 210 RPM with my saddle in the normal position and am not sure about hills.
I have a question though, when I go down hills I spin so fast that my front wheel (fork and stering as a whole) starts wobling, if I hold onto my bars tightly the wobling is wose so I just go loose in my arms, but there is still movement. What should I do to correct this? would lowering the seat a bit help this problem as well?