I hope you have a VERY large garden. Some eucalyptus trees can get very large indeed, up to 60 METRES tall, and the trouble with them is that as they get older, large bits, like entire large branches, tend to fall off on a regular basis. So plant it well away from buildings, and don't park your vehicles or a children's playground under it. Most eucalyptus are quite fast-growing trees. You won't be able to keep it small unless you keep it as a bonsai.(NB. If you don't know how big a metre is, visualise it as a yard - actually about 39 inches.)

For the home gardener, trees are best transplanted as saplings. Seeds can be sown in spring. They may germinate better if stratified for 6-8 weeks at 2C. Pot up as soon as the second set of seed leaves has developed. Plant out into a permanent position in early summer and provide protection from the cold in the first winter. They prefer warm climates. They will adapt to a range of different soils but prefer light sandy loam, and tolerates poor and dry soils, especially those low in mineral elements. Cold tolerant to about Â5°C. Overwatering will cause blisters to form on the leaves. Grow in full sun. Because they have a fairly shallow root system, care should be taken to plant trees well away from structures, especially in windy areas. A very fast growing tree, new growth can be up to 2.5 metres per year. Trees are gross feeders and can severely stunt the growth of nearby plants.

The correct spelling of the tree Melaleuca alternifola is Ti-tree, but by common usage it has become Tea Tree. Again, it's a tree - growing to a height of about 8 metres, so needs plenty of room in a large garden. Seed is very tiny. Sprinkle on top of very light potting soil and gently press it in. Keep moist by means of bottom watering or misting. Keep warm until germination. Seedlings must be protected from frost and should never be allowed to dry out. Prefers full sun and moist, rich soil. Re trimming back - you'll do that whenever you harvest the leaves, but otherwise you won't be able to keep the plant small.

Stevia: plenty threads here on Stevia. It will grow to about 80cm tall and about 60cm wide, so is quite a small shrub. See link below. Re trimming back - you'll do that every time you harvest the leaves, and whenever you chop off the flowers.

Bayberry: again a largish tree, growing to about 12 metres tall. If you want fruit, you'll both a boy tree and a girl tree. Tolerates poor, dry and wet conditions. Deciduous in most climates, though less so in hot areas.

Elder: depends on what species you have. If Sambuccus nigra, it will get to about 10 metres tall. You can chop it right back to almost nothing, and it will still grow to full height in a single season. Again, it needs a fair amount of space because of the spreading root system - it's a suckering plant, and not easy to keep under control. If S. canadensis, it's a smaller plant growing to about 4-6 metres, but it still has the aggressive root system. Not a hope of keeping either of them small!

Best method of propagation is to remove and plant suckers which appear at the base of the plant. Can also be propagated by seed, root or stem cuttings taken in autumn. Sow seeds in autumn, 1cm deep, in a very rich compost medium, covered with 2cm fresh sawdust. They will germinate not the following spring, but the next after that. They will be ready for transplanting after about one year, after which time growth rate increases. Store seeds in a warm, moist medium for 3-6 months, followed by 3-6 months in cold conditions, before sowing. Hardwood cuttings up to 30cm long may be taken in autumn or early spring, and semi-hard cuttings up to 15cm long in summer or late autumn. Once germinated, the plants grow rapidly. Prefers moist conditions and full sun but will tolerate partial shade. Prefers deep, rich soil with neutral pH, but will tolerate poor, acidic soil. Water well when fruit is forming to increase yield. Contain roots or remove suckers as they appear. Prune in spring. The plant is susceptible to aphids.

momandteacher, in Pennsylvania, you will need to keep your Lemon Eucalyptus, Tea Tree and Stevia in pots so you can bring them in when the weather turns colder. It will be like we northerners growing bay tree in a pot. Sure, in its natural clime, it grows into a tree, but we grow it as a houseplant and it stays much smaller. Ditto the three plants you purchased.

Your elderberries and bayberries can be planted outside, in moist yet well-drained soil. They will get to be 10-15 feet high and will produce lots of berries in years to come.

Beste of luck to you.

Bay responds very well to being clipped. With the eucalyptus I prefer to let it get maybe 2 feet or more high before cutting some of the branches back to the base. That way they can be dried for flower arrangements.
I've had my bay tree for 3 years now and it is about 15" high.

Oldroser, please note that Bay is not the same plant as Bayberry.

The first is Laurus nobilis (which can be topiarised), the second is Myrica cerifera, M. carolinensis.

In my response above, I have assumed the OP is referring to Bayberry, the name she has given us.