Opinions are like ar$eholes - everyone has got one. I suggest taking a good look at the various responses and choosing the most sensible advice.
1. People mentions using DIN regulators for pony to make setup more streamline.
As has been pointed out, using DIN for the sake of DIN is somewhat arbitrary. My suggestion would be to set it up with a DIN-K valve (this is a 200bar DIN vavle, that you screw an insert into to make it yoke) and a regulator that can be converted to DIN easily (e.g. Scubapro, Apeks). Set it up as a yoke and see how you go with it. If you find the yoke assembly is catching or is in danger of being hit hard enough to dislodge it, then shift everything over to DIN.
Then just buy a DIN fill adaptor - it takes a couple of seconds to screw in to the valve by hand when filling and isn't an inconvenience.
2. How often it needs to be refilled (of course if it was not used/freeflowed) ? I assume that I will need to take few breathes before each dive (normal pre-dive check) but with smallish 13CF bottle it can deplete bottle fast enough.
There's no real answer to this question other than "it depends". It will need filling every time you decide to do a dive where the pressure has dropped to a point where it doesn't contain enough gas to enable you to surface safely.
The only way you can know what this pressure threshold is, i if you work out your breathing rate and understand how to do minimum gas calculations. Do a search for "minimum gas reserve" or "rock bottom" and you'll find plenty of examples of how to do these calcs.
FWIW, my personal opinion is that a 13 cu ft bottle is too small for all but the shallowest of dive. I would urge you to take a good look at your diving profiles, and do the calculations to work out how much gas you need to surface safely from the depths you are planning to dive to - not just now, but in the future. To my mind, a 13 cu ft bottle is just about big enough for an average diver to make an emergency ascent from somewhere in the 20-30m range, but with no safety stops or much vestige of control. I'd definitely look into a 19 cu ft or larger.
3. What type of pony gauge you would recommend:
a) mounted directly on the bottle;
b) using hose strapped to the octopus;
I prefer to have an SPG on a 6" hose that bends back on itself and is tied with a loop of caveline. It's nice and clear, easy to read and doesn't get in the way. But, this only really works if you are carrying the bottle as a "slung" tank.
If you go with another form of carrying, you need to take a good look at your hoses - where they are, how long they need to be, whether they cause confusion etc. Your pony SPG needs to be very distinctly placed and recognisable from your main gauge. It's a real bu&&er when you realise half way through the dive that you've been looking at your pony gauge by mistake, and in fact your main tank is near empty when you thought you had 200bar.....
BTW: I am planning to keep it strapped under bladder wing of my Ranger LTD with regulator pointed down.
This begs a whole set of questions - firstly, are you taking the pony on every dive? If you get everything set up perfectly for the pony, then you decide to leave it behind.... does it change anything on the rest of your configuration? One of the nice things about a slung pony is that it changes nothing - it's a self contained unit, you pick it up and clip it on when you need it. It also give you better access to the valve, easy visual contact with the SPG etc. You can also unclip is during the dive - either because you need to pitch it, or because it's easier to give it to someone who is OOG than to share air etc.
Have a look at how to calculate the size of bottle you need, if it's bigger than 13 cu ft then have a look at how to sling a tank. If you can get by with a 13 cu ft, and want to carry it in your Ranger then make sure you get it right - hoses all the right length (you may need custom made hoses), things positioned so they don't cause confusion etc etc.
Also, be vary wary of having any gas mix in your pony that is different from you backgas. Switching to a richer nitrox mix is very uncool, switching to a leaner mix is less immediate but it does potentially push you into having a deco obligation due to the switch - that your computer won't know about, and do you have enough gas in a 13 cu ft tank to extend your safety stops enough to clear it????