I'd recommend putting some low value high power resistors in line with each battery when you put them in parallel for the first time. This will slow down the balancing and prevent the initial spike of current. This method is slow but easy. After they have settled, take the resistors out. Be sure to run a few quick numbers to estimate the power your balancing resistors will put out, and how long it will take.
Since our batteries are charged with 2, 4 or 6 amps, I'd multimeter verify that they are all within a volt and then put half ohm or quarter ohm 1 watt resistors in there. Since we have 18 Amp hours, I'd plan on waiting a full day. If you do the math, I'm being too conservative, but why rush?
At this point, you should treat the pack as one unified battery, and avoid taking them out of their parallel configuration for anything. Otherwise, you will need to balance again.
As for diode-oring them:
From your description, I assume you are using the pack as a back up for a solar array. Charge during the day, discharge at night. If this is the case, be very careful about where you put your diodes. Frankly, I wouldn't use them in this case.
If you still want them :
I'm not a huge fan of steering diodes because of their voltage drop. Even a big schottkey will drop a third of a volt. To reduce that, you could use an ideal diode controller like the LTC4352, tps2413 or ISL6144. If your currents were lower, you could use an integrated ideal diode like the ones from picorpower. Since you have 7 of them, you could roll your own like
this.