So I don't have lots of money and just recently the bb on my vintage tube shifter 2x6 road bike broke and I have to make do with a slightly too long bb and mtb cranks because it's what I have. Anyway before I can replace the bb and cranks, I'll have to make do with the big chain ring in the front (48) and the 4 heaviest gears at the back to prevent cross chaining. I find myself going usually on the two middle gears. When stopping at a red light, I don't change gears, but just crank it up slowly while standing until I got to my desired cadence and then sit, though sometimes I got a little excited and sprinted away. I've heard that climbing in too heavy gears can damage your legs, so is this practically the same? will I damage my legs doing it this way? I don't feel pain or anything, but just in case.
No, there is no inmediate danger to your knees. Most dangerous thing can happen is the chain snaps and you ram your knees on your tee, or frontflip over the bars while still.
Most Bmx riders use a equivalent cadence(I think this is the word) wheel to gear ratio, and almost every one rides standing up. Because part of the effort is made by gravity and your weight.
Just adjust the derailer in the front with the stop nuts and ditch the cable, so you can use your bike with a smaller sprocket. This is the poors man downhill chain guide.
Depending on your body size, leg power, and hills in your area, you can snap chains and wrench axles out of the dropout with too much torque.
As for your legs, BIG gear ratios are a good way to get BIG leg muscles. There's a reason all those fixie punks favor cutoff jean shorts. But, as with any repetitive exercise, you can strain your knees if you're not careful.
I did this to myself, but the solution was to fiddle with the ratio a bit and listen to my body. Recently, I've been learning about the ratio of hamstring to quadriceps strength being a major factor in knee injuries- having comparatively weak quads will leave your knee unsupported.
So get some clips or powerstraps and you'll be rocking it.