Stock sooner, after adding the plants: you'll need to keep the filter bacteria alive and some plants don't tolerate ammonia very well.
In terms of stocking, I would stock tiger barbs first, loaches 1 week later and Kribs a week after that.
I recommend that you ask your LFS to look even now for a paired up pair of Kribs, it is not nice on the fish when two individuals are not given a choice of partner, and that you spend some time observing them at the LFS before taking them home. Picking two random fish can cause problems because they may turn out to hate each other.
You won't have a bio-filtration problem, an external can take a LOT more bio media than an internal which is rated for the same volume.. when someone came up with the length per volume guideline, internals were more popular due to the cost, I suspect, so it was probably aimed at them.
Rainbowfish are all schoolers, same goes for all tetras, rasboras, barbs and danios.. so the usual "at least 6+, or even better, 10-20+" recommendations apply.
Have you thought about the diet for the fish?
I recommend a generic (omnivore, usually mix of red, brown, yellow and/or green) tropical flake and a generic (omnivore, usually brown and red) granule and/or small tab food for the barbs and Kribs; sinking meaty pellets for the loaches; frozen tropical quintet/quartet as treats. It is well worth buying the dry foods in the smallest tubs, at least the first time around, especially if you are buying small fish, so will need finer foods to start with. One option would be to ask your LFS if they have any samples of flake food, etc, because a sample jar is usually 50-100 ml in volume and one of those should last you anywhere from 6-12 months, depending on how you feed. I do like Hikari foods for my fish, but to be honest, I have never used a dry food which is a good all rounder, there is something lacking regardless of which manufacturer one uses or how much one pays, so I very strongly recommend buying foods from different manufacturers for a more complete diet (for example, flakes from one, tabs from another). In terms of number of different dry foods, I think two types is a good place to start for now, and right now, I have about 15 different types (various flakes, pellets, granules, tablets and rods) just for the mid-water schoolers, but there's no reason to get as extreme as me.. I'm just trying to use up the ridiculous number of samples have! Every time I see an offer online, I take advantage of it
For feeding regime, I recommend you start with once per day, everything gone in 30 seconds, and increase as needed. Loaches will probably need at least one pellet each. Frozen foods are a good option once per week, again, can be increased as needed up to 2-3 times per week. Once day per week should always be completely without any food. Do keep in mind that growing fish can do with more regular feeding than adult fish, to encourage good growth. If you feel the fish need more food, increase the feeding frequency, not amount of food given, their stomachs are quite small.
p.s. did you get a timer for the lights already?
/fishprofiles....Rock_Kribensis/ which seem pretty special but the reference to killing each other came as a bit of a shock! Do you have any recommendations within the Kribs family as being hard water happy and preferably unlikely to kill each other!
