Forces at Peking

8 nations were represented in the legations and the following forces were involved:

Autro-Hungary: 5 officers and 30 men
France: 2 officers and 45 men
Germany: 1 officer and 51 men
Great Britain: 3 officers and 79 men
Italy: 1 officer and 28 men
Japan: 2 officer and 24 men
Russia: 2 officers and 79 men
USA: 3 officers and 53 men

There were also 75 men who had military experience.

Lastly there was second group of 50 civilians who called themselves Thornhill's Rough.

Heavy weapons included:
Austrian Maxim Gun
British Nordenfeldt gun (which incidentally jammed every 3 to 4 rounds)
Italian 1 pdr quick firing gun
US Colt Machine Gun

Later on some Chinese workers found an old Anglo-French rifled cannon. The Russians had some old 9lb artillery shells which they had thrown down the well to stop them getting in Chinese hands. The Americans got the cannon working, the Italians supplied a working gun carriage to place it on. It was called many names but the name that stuck was the International Gun.

In 20mm there are 3 manufacturers that I know:
Kennington Miniatures
Orion Plastic Boxers
Rec Box Plastics that cover a host of colonial troops and some of the relief troops too.