According to some databases, (see 戚 vs 歴), the order of strokes of the upper part (厂) of the kanji 戚 is different from the same part in 歴. They may have been some historical reasons, but as they look identical now, one should not expect from the writer to write them any different if they are in fact the same. Is it acceptable to write them the same way, or is there any reason why we should not do it?
Both Kanji are expected to be written with different stroke order because there is no "厂" in the Kanji 戚.
This answer is based only on my own experience.
All Kanji similar to 戈 have the vertical stroke as the first stroke and after that the horizontal one. For example, 戚 成 歳 蔵 城 減 感 to name a few.
On the other hand, all Kanji similar to 厂 have the horizontal stroke as the first stroke and after that the vertical one. For example, 歴 厚 原.
What looks to you as a cliff radical 厂 in 戚, is in reality a vertical stroke attached to the halberd radical 戈. Therefore, in order to write 戈 at once inside the kanji 戚, it is first written the vertical stroke / and after that, 戈 which starts with the horizontal stroke. But it is not the radical 厂.
Also note that the look-alike "厂" from the kanji with 戈 are always shorter than the actual 厂 in the kanji with 厂. They do look similar, but not identical.