The 6500k bulbs will work fine, most people prefer it because they are pretty close to noon daylight rather than the yellower cool whites that are more common.

Tap water with 0 kH and gH? wow, that's unheard of.

Leave the lights on for about 10 hrs a day. You don't have to increase it later when you get more plants, the demand on lights are the same no matter how many plants you have.

Crypts are great low light plants as are java ferns, the vals and hygros are a little more demanding but you should have enough light.

If you want to prevent algae, have lots of plants right from the beginning. Do not put a few in at a time and wait for them to grow out. If you have alot of hygros already great, if you only have a few stems it's a good idea to get more. Other plants you can use are anacharis and hornwort.

You should be fine without CO2 or Excel, growth will be very slow is all.