Welcome to the hobby, I say hobby it is more like the Guild though as you will find out once you get fired up and going.
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To start with I can only suggest that you do plenty of reading up on things before you start, this way you will know what your looking for, what to expect, and what your seeing when you see it. I know that sounds somewhat vague, but as you go along you will understand as things WILL come up that you have no idea why they did that. As such, here is a link which has a TON of info on it which is divided up in a good easy to follow format.
index to all of the articles/authors and handloading pages on lasc.us
On this site it has most of these articles posted in a PDF format as well so you can download them and save them to your PC or print them out into a very nice handbook for future reference. I found a large handbook in a 3-ring binder at work which was being trashed. It had a bunch of dividers in it already so I junked all of the stuff that was there and printed the Fryxell book and a lot of the other items and put them in it for quick reference. It has helped out enough times to have already made it worth it.
Ok ok back to your OP, the answer is yes and no...
First things first, your going to find out that with cast things sort of run like a deck of cards, Fit is King, Lube is Queen, and the other components fall in line after that. If your barrel is smooth, and you have a good fit you can fun softer alloy faster than if otherwise. However, your lube must also be up to the task as well. Most are but not all. Hardness is a bit overrated until you get up into trying to run cast in rifles at jacketed speeds. It is the pressure under them which sets the give and take.
If your loading soft cast in your 357 with a high pressure load, then no the soft stuff isn't going to handle it. However there are loads in which the pressure isn't quite as harsh that will allow it. So it is a balancing act, of which alloy you can run at what pressures. Then you have to have a lube which will handled all of that as well.
Personally I use the White Label Lubes, they are great lubes and don't cost a small fortune. I use both their Xlox 45/45/10 blend and their Carnube Red. The former for loads running up to around 1000fps in all calibers and the latter for everything else. That said I use the 45/45/10 in my 454 loads with GC'ed straight air cooled wheel weight, and have no issues up to around 1600fps.
Anyway, if you read through the link above and those which it lists you will get a much better handle on where your going as well as where you will need to be once you get going. There is also plenty of info in there for blending your alloy up a little harder and what results will or should be before you start. This sort of info is good as with most blending you need to start in small amounts and move up slow. Sometimes adding a little of this, can change a LOT of that. When you start to blend an alloy keep detailed notes. Write down everything from the amounts your using of each component, to the temp at which you smelt it all together and in which order you added things to the pot. Trust me when I say, it really sucks to blend up something great, then seven to nine weeks later when you test it and find out it is great, not be able to remember what you put into it, and have to start all over. Also try your best to keep things simple as you can, you don't want to have to purchase tin, or antimony, if you don't absolutely have to. If you can scrounge up some 95-5 or similar solder that works great. Antimony and/or arsenic can both be found in chilled shot and neither takes a lot to harden up a softer lead. But it is a test and see thing, so remember a little at a time.
One more thing before this needs a second post to continue,
If you try out your alloy and DO find you need to make it harder, and you have read the info above and find it somewhat confusing. Over on the Castboolits site, look down to the Lead and Lead Alloys forum. In there you will find a bunch of sticky topics. One of them is titled Alloy Calculators. Go there and download the one posted by Bumpo. It will help you to work up a decent alloy. It uses pounds for all of the calculations so you might need to bring up your calculator and I also use a freeware program called "Convert" which helps to break everything down from pounds to ounces so I can weight it up once I am finish with the alloy I want.