Abby's been walking outside for a while now. She's not one for sitting in her pushchair for long periods to be honest.
At nursery they go outside every day, my parents walk her down to the highstreet once a week and we allow her to walk whenever we go out with her. She also has access to a number of gardens and loves going with Nana to throw corn for the chickens.
From a safety point of view it is essential that she will hold our hands. If she refuses she goes back in the pushchair temper-tantrum or not. We do allow her to run around of course but practice calling her back regulary for another bit of walking holding hands. I have owned dogs for a while and used the same technique with them - bit of freedom, call back, praised, walk at heel, freedom etc. :\) I have had a couple of issues with her running off the moment we get to the car and I have to let go to put bags etc in the car. Obviously this is extremely dangerous in a car park so she is now told to "hold onto the car". This works quite well although she does just hug the back tyre and gets a bit mucky. Not very hygenic but 1000x better than being hit by a car!
Abby does have reins but she learnt very quickly that if she leans into them then it's a ready made swing. My parents seem to use them much more successfully.
As for shoes I have some walking boots in a 4.5 and wellies in a 4. However Abby is still a tiny size 2 so is restricted to her cruising shoes. I can't believe she still hasn't grown out of the shoes I bought in October!

You can definitely buy wellies in half sizes if you look around but I would go for the size 5 with thick socks as they will last a bit longer. It's not although they wear them all the time so I wouldn't worry about blisters etc. I love these:
http://www.cradleandall.co.uk/raincoats-wellies-c-6.html
The pirate ones are fantastic for a boy.
H xx