I'd like to know what the plants are (some are more susceptible to scale) , what type of scale it was, how big the plants are ... and what you are now going to put them amongst...and what you've been treating them with... but.. by way of a general answer(s)
1. The only way to keep those plants, and the rest of the garden safe, is to keep on top of the scale.
You need to know a bit about their life-cycle. They can reproduce very fast and at first stage they are mobile and called "crawlers" and they go off looking for a nice juicy spot to anchor themselves. At this stage there are thousands of them - almost invisible to the naked eye - but they are at their most vulnerable. Light chemical sprays that reach all parts of the plant, will usually do the trick.
After that stage they begin to settle and may be oval or flat or raised but often simply green or whitish or opaque and it is really hard to detect them unless you know what you are searching for. At this stage also they can be easily killed by chemical treatment, even by gently dislodging them with a cotton bud.
Once they anchor, they set that hard shell and at that stage they are harder to treat. Type of treatment depends on the plant, and where it is - oil based treatments "drown" the insect but can cause burning to leaves in outside areas unless used carefully. Also some folk don't like using pesticides inside the house. But at this stage the scale are pretty "immobile" so can also be removed manually with a toothbrush or a toothpick, just very gently popping them off the plant.
Anyway you know what to look for and to keep up the inspections every few days and even so, the treatments every 2 weeks at least for say 2 months, to make sure you have got them as they have a very fast reproductive cycle and the eggs can then still remain ready to hatch.

The second thing is to try to keep your newly planted out things a little apart from other plants so there is no "bridge" (leaf to leaf) for the scale - or ants - to climb over while you are eradicating the scale.

You must get on top of the ants, as they are attracted by the honey-dew that the scale excrete, and they actually "farm" the scale and move them around plants. If you don't object to more chemical help, dust the plants and the immediate soil with an ant-killer (not a spray) - I have found Hortico Ant & Roach dust very fast and efficient at killing ants on the surface. Depending on the type of plant, you could also try smearing vaseline (petroleum jelly) on the trunk of the plant from ground level to a few inches up - this also deters ants from trying to climb into the stems.

2. Yes, the ants do add to the problem. If you were only keeping the existing plants, then I would say you should treat for the ant infestation also and usually you can do it by (a) plunging the pots in a bucket of water to above soil level for say 15 mins till all air-bubbles come out, and the ants will usually evacuate...

Well that was a good read. The plants are 2 citrus trees that are quite small. I will be putting them amongst some ginger bushes and a franjipanni and some native bottlebrushes and there's some bromiliads and pineapples in there too... It's just a miss matched garden . Anyway they had the brown dotty looking scale... Sorry I don't know the technical term sorry but I have been using an oil based treatment to get rid of them which seems to have worked though I will keep an eye on them and keep spraying if need. The ants seem to have gone actually. They used to pile out every time I watered but there's not one in site now... Wierd... I'm hoping they are safe to plant now...

Yeah sorry if I got carried away. Citrus are very susceptible to scale - just keep checking under the leaves cos new ones are so pale you barely notice them. The other plants you mentioned don't sound like they would be susceptible to scale.
I didn't mean to knock oil based treatments like white oil it's good but just have to avoid too sunny a day outside or you can get a bit of leaf burn.
Good luck, your fruit trees should rocket away now they are liberated!!!! Just make sure they don;t get swamped, they do need plenty of sun to do well.

Yes I was worried about the sun when I first did it but it doesn't seem to have been effected. I hope they do well in the garden... I have had plants that have died after being put in the garden... Hope that doesn't happen ... Anyway thanks for the scale advice everyone :)

get rid of the ants and the scale will die off.

len

Here is a link that might be useful: lens garden page

Ants hate Cinnamon. Sprinkle a little around the plants.In my experience it works every time. Good luck.
chas