How on earth can a full-grown man (Ross) be over powered and captured by two small and I mean small aliens (Minotron and Minador)?

I haven't seen this ep, but is this the Ross from Space Magnet?

Yes, that wimp was from Space Magnet, Plant Man From Space and Space Immigrants.

Ah, Ross McBighead.
Did good ol' Chuck put in any more appearances?

Not as Chuck but he was Al Stomper from Space Monster.

Did anyone else except the main lot (Steve, Venus, Matt, Zero, Zoonie 90) and Ross, Jock, Boris, Grizelda or the Subterrains return as the same character in another ep?

Ken Johnson he was in
Space Monster
Flying Zodiac
Then he was Johnny Jackson in
Convict in Space
Forbidden Planet
Space City Special
Then he was in Stingray
Tom Thumb Tempest
Deep Heat
Titan goes Pop and
Aquanaut of the Year

I have Convict and Space and Forbidden Planet..... is Johhny Jackson the newsreader guy? If so, then he is in Echo of Danger too.

Yes Jackson is the news reporter
I didn't see him in Echo of Danger what part of the episode did you see him?

Wait I know he is out side the WSP/WASP conference isn't he?

Yep.
And then later in the Blue Lagoon bar.

The episode features the last appearance of Ross. He probably got fired from Space City for being a weakling and giving in to the enemy that are as tall as a toy doll (If not smaller)

Who came up with the brilliant notion of colonizing a planet that does not have a breatheable atomosphere?
Was it the same person who decided to send out a bunch of immigrants before the atmosphere problem had been cleared up?
The same person who decided to make a medical doctor the designated pilot for the venture, rather than a trained astronaut?

I can't imagine that the plant life that would grow in a non-oxygen atmosphere would be likely to appeal to humans.
They seemed to have made Venus the pilot in order to take slams against women in general, via Jock the haggis-eater. Venus certainly lived down to his expectations. I don't know how long the journey would have taken, but it must have been a while, and in that time she never noticed that her drug cabinet was standing open and empty. BTW, that was an awfully tiny cabinet to supposedly contain all the required drugs for a colony expedition. Having discovered trouble, Venus is incapable of making a command decision. ("I wish Steve were here; he'd know what to do!")

They never showed the little flap-footed aliens sending Venus the necessary drugs for surgery. They just told her to get on with the operation, and gleefully noted that none of the people could get out of the ship without oxygen.

Just why are these little guys considered so dangerous? And if they're so superior, why do they need slaves?

Those little aliens looked like cartoons that had become three-dimensional. Their voices matched their cartoon image perfectly, not to mention the way they would bounce up and down with glee.

Ross was a Captain in Space Magnet, he's a 'Space Lieutenant' here. No wonder...Zoonie'd be a better officer than Ross.

Perhaps we'll see Ross in the background, pushing a broom at Space City....

BTW, those little aliens reminded me of the little alien that used to pester Fred and Barney on "The Flintstones." Fred's friend didn't have that malicious expression, but he had the same casual assumption of superiority.

They reminded me of one or two Disney characters...the Mad Hatter, at least.

Presumably the space ships have artificial gravity installed, because otherwise the people would either be floating around inside, or forced to constantly wear those heavy-soled boots. Yet, during the operation, Venus got rid of her used instruments by simply letting go of them and letting them float to the ceiling. (And how did she keep the unused instruments on the tray?)

Does the artificial gravity only work in space? If so, then they did a pretty poor job of design.