I personally would be far more careful. Two batteries aren't going to be exactly the same - not now and certainly not later, as they age. Two things result from this:
None of that is good. You can somewhat alleviate the problem #1 by using diodes. But you will be losing a lot of power on them, especially if the current is high (the forward voltage on a Shottky diode will be around 0.3V.) At 8A you will lose 2.4W per diode. This is unwelcome in itself.
The problem #2 may result in bad batteries, quickly. Batteries are generally delicate devices; more so as they get more and more sophisticated. A typical notebook battery has an integrated charging controller inside. It's because they will catch on fire if overcharged (and it happens now and then.)
To begin with, the whole idea of storing energy in a battery is not that great. Some people already commented on that. A battery that cycles so often won't last long. Note that car batteries and UPS batteries are used lightly, and kept charged all the time - that's how they get some reasonable lifetime out of them. Cell phones and notebooks have this problem - the number of full cycles of a LiIon cell is around 500, and then it's dead.
There are solutions that work. The most effective one is to use the electric grid as your battery. When you produce you send the energy into the grid, and the meter spins backward, giving you a credit. You can use that credit later on.
But, for purposes of the argument, if you want to use several batteries then the best way to connect them is in series. This is because batteries are charged by current, not by voltage. Voltage varies between cells, but it doesn't matter until the differences grow to be too large. Your 12V battery already contains several cells connected in series, and if you connect another box of them also in series then it won't change a thing, from the POV of one cell.
On top of that you need to build a small computer that manages the battery (regardless of how large it is.) This controller has to measure the charge (in coulombs.), this is usually done with an integrator. The controller then figures out how much energy can be transferred into or out of the battery, and allows that to happen. Since the voltage of the battery changes as it charges or discharges, you need a voltage inverter (DC-DC or DC-AC) to regulate the output. You will end up with a good part of Prius electronics.
If you want to install a solar (PV) system the best way to do it is by using panels and inverter(s) that are commercially produced. If you simply want to experiment with things, this particular setup is not the best. Talk to your local hams instead, they will throw a bunch of far better ideas at you, and on top of that they will advise you what to do (and what not to do :-)
Even lead acid batteries require a sophisticated solar charge controller. (~$120)
Even deep discharge rated batteries will only last around 400 charge-discharge cycles at an 80% discharge level.
Suggest you go to this site and do a lot of reading: http://www.windsun.com/Batteries/Battery_FAQ.htm
For what it is worth I generally agree with what you said (Greysard) with one exception.
It is a very bad plan to wire batteries with different capacities in series. The smaller capacity battery will charge first and then be overcharged to reach full charge on the larger capacity battery. In addition when they discharge the smaller capacity battery will discharge first and can be reverse biased by the larger battery. Very bad idea.
So, do not wire different capacity batteries in series.
It is not normally a good idea to wire batteries in parallel for the reasons you stated. And if you do so, they should absolutely be fused between batteries for safety. In addition the batteries should be physically close together so they are both the same temperature. Temperature affects the output voltage and output voltage differences between paralleled batteries means large currents flowing between batteries.
Honestly, I would not wire batteries in parallel unless there’s no other practical solution. And if you are going to wire them in series they need to be the same capacity and about the same age and then it is okay.
My 2cents...