I would discourage you from being so firm on this. One reason is that some plastic tubes are nigh useless without their caps due to their oblong nature. Google Image search 'plastic test tube' and take a look at the early hits, all common (as common as these are, anyway) items. It can be argued, then, that the cap is an implicit component of the test tube. What you've suggested is tantamount to confiscating the plastic container in which pH paper may be rolled. It's not explicitly permitted by the rules! However, you don't confiscate it because it's understood that 2.a.iii. permits it as part of the pH paper.Be careful here... This issue is not with the plastic tubes, but with the caps. The rules specifically do not say that any type of stoppers or caps are allowed, which means they are not, and should be treated as contraband.
You can't get around this problem, note. 2.a.i. is written such that it disadvantages teams whom don't pack each and every one of those items. Here's another example: there are many shades of conductivity tester that you will see. Underneath the umbrella of 'the rules', there will be teams with better (commercially-produced) instruments than others that, yes, could and does give them a competitive advantage over others whom either spent less money or built their own. It's part of the game.If you have caps for your tubes, while everyone else does not, you are exercising an unfair advantage over the other teams.
This depends on how many points your test contains, but remember that you can only penalize a maximum of 10% for all unauthorized items. Put another way, if you have an 100pt exam and a team gives you a Crime Busters box at check-in (which happens), even though you obtained a lot of junk in the process, that's, still, only -10pt, not minus five points times the number of microscope slides, cover slips, magnets, and whatever else they managed to sneak in there...I'll be running CJAP at Regionals in a few weeks, and teams with unauthorized equipment will be penalized 5 points for each item I find in the kit that is not on the approved list, whether they have used it or not...
READ THE RULES CAREFULLY!
Anyway, hypothetically, know that if my team came to you with capped plastic test tubes and had their caps confiscated with a point penalty that you would be visited by the arbitrators shortly after the event. And, I would wager that you would be hard-pressed to defend pulling a 'Gotcha!' on thirteen year-olds over a gray area related to safety. This is not the same as if students had thermometers, voltmeters, pH meters, or (oh, brother) pencils in their box. Those items should be rightly confiscated. This is more akin to confiscating the case surrounding the pH paper.
Here, let's expand the discussion for the sake of case example. Have you read the FAQ posted recently? I submitted that one over a disagreement with my own team as to whether they should bring graduated cylinders or not. I argued that bringing them when others don't offers competitive advantage since they're a volume instrument (and hardly 'small containers for mixing'...), covered by 2.b.vii. Well, the National level hath spoken, and I was wrong. Graduated cylinders are permissible under 2.a.i. I admittedly don't like it, but it highlights the laxer nature of this and how, despite the beginning of section 2., all of this is framed by, I suppose, the first General Rule. And, I like to think that that's not just 'Science Olympiad policy' but, rather, student-centrism, which I would hope is our goal here!