Containers for pickling onions and how long to leave before eating?

I've grown some barletta silverskin onions for pickling. A small number of them are ready but not enough to fill my 1/2 litre storage jars. I have an old jam jar in the cupboard -- if I sterilise this will this be okay to use? I read about the lid needing to be plastic coated on the inside because of the vinegar and I think it is.
This is my first year pickling and trying to make sure I do it right. It's not the same with sterilisation/preservation as the chutneys we made last year as the beetroot and onions aren't hot when they go in to seal the jars. And when we tried making jam and put it in the jam jars the heat sealed it and sucked the popper down. But this doesn't happen with the cold stuff.
I've done a jar of beetroot -- when I open it again is there a time limit for eating it? Also does anyone know how long you should leave it before eating it? My sister has a book that says it needs to be left for three months, whereas in my book it says you can have it after a week and that it's at its best for three months. Which one is right?! And is the rule for onions the same?

My mum does the same thing, Grannieannie. I'm not much of a pickle eater but she does loads every year. The old glass-lidded kilner jars are perfect for this too, especially the 2 quart ones. Not sure if you have these in the UK but they go for a song at auctions and yard/boot sales here since they need rubber rings for true sealing and few people will bother these days.

I believe the 2 qt size was made specifically to do large batches like pickles. Most homemakers had them, maybe a dozen jars, but rarely more unless they had very large families to feed.  I used to have one but it somehow 'disappeared' so I'm on the hunt for another as they are very handy for dill pickles that don't need water bath sealing.  They're now getting harder to find as crafters like to use them or people simply bin them. The 1 qt sizes can be found anywhere.

I know the size you mean, Val. Our modern mason jar producers used to do a 2 qt jar but don't make it now, and any of these are as rare as hen's teeth. Sadly, I didn't think ahead and purchase a box for the future. I have to deal with a neat freak who constantly nags: if you're not using them, then get rid of them.
Not sure why he's so keen on empty space 

I re-use the rubber rings - you can tell when they are past it as they perish. Just clean them with a little washing up liquid. Onions I tend to give a couple of months but it is a matter of taste. Beetroot, sliced ready in a month and whole baby ones I give 6 to 8 weeks. Beetroot is generally best eaten within about 3 to 4 months anyway.

With these cold pickles (and chutneys that you pot while hot, too), the length of time is to do with allowing the flavours to develop.  However, there is a fine line between maturing and going soft!  

With the onions in particular, you can either pour the vinegar over them whilst it's still hot and seal it straight away, or leave it till cold before doing so.  If you do it hot, they'll 'cook' a little and not be so crisp, but they seem to take on the flavour a bit sooner.  I guess it's a matter of personal preference - I prefer to bottle them cold and leave them, but if you add plenty of spices in the first place you can enjoy them within a couple of weeks.  

This is my favourite recipe for spiced pickled onions/shallots in balsamic vinegar - this mix would overpower your little silverskin onions I think, but worth keeping for future reference!

For every 500g of onions or shallots, use 150ml balsamic vinegar and 150ml cider vinegar.  Bring gently to the boil with a couple of tablespoons of muscovado sugar and a tablespoon each of juniper berries, peppercorns and allspice berries plus a couple of bayleaves, a cinnamon stick broken into pieces and half a teaspoon of chilli flakes (more if you like them hot!).  Stir till the sugar has dissolved completely.  Take off the heat and leave till cold, then pour over the onions and seal.  I've just opened a jar I made last week, and although the flavours aren't quite as full as if they'd been left longer, they're still pretty damn good (even if I do say so myself)!!